Compensated channel selector



W. D. HOUGHTON COMPENSATED CHANNEL SELECTOR 5 Sheets-Sheet l Filed Aug. 4, 1945 5 Sheets-Sheet 5 INVENTOR /u/AM .0. Haug/TQM @Y )f/W,

ATTORNEY Nov. 28, 1950 w. D. HouGHToN 'coMPENsATED CHANNEL SELECTOR Filed Aug. 4, 1945 .N h an@ M 7 m 1 5 y 8, t W, n 1 9 TMW N 3 .n mv W H 14| 3 4 5 N Z/ R by Sw 4 H H u.. E m 2 t V M T e W A M l M s 1 1 5 .l l m l' IL.' O nunH w Il Il l l N m l I L l l| w m l l w H l!llnlmnwm I .ill D l l.' I.' D m M J Il |.l la 1| W m nnnl IJ m w23.- Jr l1 W2 .lr I.' J .lll Il ffl E ILII u 5 HVUVH 4 0 w 1|. 4 8 O 0 O 0 2 M SQWQSM W W d f w m N F wb Patented Nov. 28, 19.50

UNITED STATES OFFICE ooMPENsATED" oHANNEn sLEoToa WilliamD.-Houghton, Port Jefferson, N. Y..a'sA

signor to Radio Corporation oiAmerica-a cor-` poration of Delaware Application August 4, 1945gSe1-ial'No; GSQQ?" 'syst'en'n a plurality of" substantially identical channel selector circuits having their inputs con# nected in electrically parallelrelation-and so designed that their different voltage outputs are-of substantially the same magnitude, asa result ofv which the operating positiorrof any one channel selector circuit relative'to different values of input voltage can be changed by a simple bias adjustment; without the need formaking other adjustments to compensate for the' changel inl bias.

A' further object is to provide, in' almulti'plexi'` system, a channel selector circuit which furnishes a constant output voltage' over an appreciable range of" cathode biases, andV which"V requiresv no additional' adj ustments'A when i itsv bias` is changed 'over' sa'idrange.v

The channelselector circuit oft'ne inventionisihereinaiter described with particular'refer;` en'ce to a pulse type multiplex transmitter; but it` shouldx be' understood that the channel selec torfof' the invention canbeused also in the re ceiving end of the multiplex system and wherever there is need for suchla selector circuit.

In i the accompanying drawings:

Fig. 1 illustrates, in box form, thecomplete transmitting system for a1 pulseftype multiplex communication system in which'the invention-is' employed;

Figs'. 2 and 2a taien together schematically il`-- lustrate the circuit details of the transmitter of Fig-1 1`;

Figs. 3A and 3a are a series of`curves graphically illustrating diierent voltage wave forms appear-V ing in different parts of the transmitter;

The transmitter' of Figs. 1 and 2` is used'v in anl eight channel multiplex system and'utilizes short pulses of radio frequency energy which are `time displaced by modulation. For multiplexingpln-vposes', the pulses correspcnding'to the-:separate channelsV are separately and consecutively generated at a' xcd. repetition rate Whichwillilie called? hereinafter the synchronization rate; con-f. responding tobai fixed time interval to` be called:` the synchronization' period.. Initliis'ftransmitting: system'. the syn'chronizationf` rate is a 10.` kc; andi the corresponding period 10u pse'c; (microsec'eonds). Apulse;occursioncefeach:synchronizationi periodi. for each charni'el... The individual rates: and. periods are. consequentlyfthasame.:andequal to; thev synchronization ratef: and'. period. Each-` channelpulse occursatiairate of. 10 kc. and the,

separationbetweenladjacent pulses-in eachchane nel. for anunmodulated condition is 100. aseo.; Because the;v unmo'dulatedz. siga nal; pulses are. similarly` locatedlin each chane nel, they; are;.,therefor.e; A about: 1 1.1 microseconds.-y apartiin the commonioutputf. circuit; The; pulses: i

or. microseconds;

in :each channelcan bexm'o'dulated' teased-(peak modulation), thusileavinga: guard space between` pulses from succeedingchannels of aboutll aseci.. This'. guard' space: is necessaryy to.: reduce crossmodulation eiects. The: pulses. from other'. channels occur. in the interval .between adjacent pulses The' synchronization from any one. channel; pulses occupy rvthe ninthiinterval of'l1:lmicrosec: onds. The. output. pulses from the'ichann'elsare, of constant. lengthandl the timebetween twofad jacent-pulses.ismeasured from the leadingedgesi.

In Fig; l there' is shownla'gikcrcrystal oscilla@ tor. producing. pulses! off current Whiclrfeed; into and' lockiby injection a .Qllkcshortvv pulse oscilla: Theshort output pulsesl from the. pulse.

tor i2. oscillator I2V are. applied to av counter or step'- Wave generator hi".` TheVoltage:wave-iorms.fr0m;. thefapparatusfl; I2 kand lfd are illustrated bythe:l curves' 9, l'l and I3:,.resp'ectivel3,f, appearingfim'emediately above the equipment.

The counter M provides two outputs, onegof which` is the step. Wave lwliich is'supplied to: the coupling amplier I6 and the other of which; isn a syn'chrcniaation pulse occurring once for:Y

each step Wave cycle and which. is appliedvia lead lliA to a: synchronization'pulse generator 30..

The function of'thestepwave I3, which is ap"-A plied to thevcouplin'g amplifier and then to the' different channels over lead lil; is tor time the mean occurrence of. each'. channel pulse; output` of the coupling: ampliiiervv I6 is applied" to a connection I'k which iscommon'to .allfthefv channels vl toi 8; inclusive. Y

All channels are"Y substantially. identical; and

each includes in th'eordr named; a1channelse-. lector' I8,vv normally noir-conductive; a saw-tooth' generator 20' controlled.` by' the output of the.'

The:

inputs connected together in electrically parallel relation.

The channel selectors are differentially selfbiased and each channel selector is normally biased well beyond the current cut-off condition. The bias of each channel selector is so adjusted that the applied step wave from the coupling amplier I6 causes current to flow consecutively in the different channel selectors. One channel selector conducts for each rise of voltage in the step wave I3 up to eight, which is the number of channels. Each step rise in the step Wave is great enough to insure that during its occurrence the current of the correspondingly biased channel selector shall be driven rapidly from beyond the cut-off condition to a zero bias value. Once a channel selector starts to conduct, the current flow therein will continue until the end of the i synchronization period, when the input voltage to the selector drops to zero at the end of the step. The outputs from all channelsl appearing in leads 29 flow in a common lead 3l to differentiator and clipper circuits 32 and 34 from which pulses of shorter duration are applied to a power amplifier 36 whose output controls the production of radio frequency pulses from a magnetron oscillator 4i). The very short duration output pulses from magnetron 4I! which may each have an effective duration of 0.3 aseo., are fed to antenna 42, from which they are radiated to a remotely located receiving terminal (not shown).

The synchronization pulse generator 3D which receives a pulse over lead l5 from the counter I4 at the end of each step wave, produces a pulse at the end of each step wave which is supplied to the differentiator and clipper 34 and also fed to the amplifier 36 and magnetron 40 together with the channel pulses. There will thus be eight consecutively appearing pulses from the eight different channels followed by a synchronization pulse for each cycle of operations. For the unmodulated condition, all of these channel pulses and thesynchronization pulse will be separated from one another by a spacing equivalent to 11.1 psec. It will thus be seen that the synchronization period of 100 aseo. is divided into nine equal intervals by the step counter or step wave generator I4, and that all of these pulses are similarly located in each one of these nine equal intervals and similarly spaced apart for the unmodulated signal condition.

In the schematic illustration of Fig. 2, the circuits which correspond with the circuits of Fig. 1V

have been given the same labeling and reference numerals. The 90 kc. crystal oscillator II) has an electron coupled anode circuit and supplies pulses of current to control the 90 kc. pulse oscillator I2. The pulse frequency from the oscillator I2 is normally determined in the main by the time constants of R and C, in the absence of the applied synchronizing Wave from the crystal oscillator. In practice, the values of R, and C are so chosen that the frequency of the pulse oscillator is slightly lower than the 90 kc. crystal oscillator I Il. Transformer TR is a low inductance, tightly coupled arrangement. The sharp output pulses from the 90 kc. pulse oscillator I2 are each about 1 aseo. long and have about 400 volts amplitude. These pulses are applied to the condenser CI of the counter circuit I4. This counter circuit or step Wave generator comprises a condenser CI, a pair of diodes DI and D2 (which may be arranged Within a single evacuated envelope or comprise two separate tubes as shown), a condenser C2, and normally non-conductive triode vacuum tubes A, B and E. Condenser CI is of much smaller value than condenser C2 and in the present case (where a step Wave voltage loutput of nine steps is desired) these condensers may for example, have a ratio of sin C'2-20 For the positive rising edge of a pulse from the pulse oscillator I2, diode DI will not conduct, but D2 will conduct; and hence the circuit will appear as though the two condensers CI and C2 are in series -to ground. Hence 1/zu of the total input voltage appears across condenser C2, for the foregoing assumption that CI has a size which is 1/20 that of C2. On the negative falling edge of the pulses from the pulse oscillator I2, diode DI will conduct, but diode D2 will not conduct, leaving unchanged the voltage on C2 acquired during the immediately preceding rise of the pulse. The voltage on CI will be completely discharged through DI during the negative drop. During the next positive rise of the applied pulse, the condenser CI will be recharged through D2. Each time there is a positive rise in voltage applied by the pulse oscillator to the condenser CI, there Will be an incremental increase or step-up in voltage on condenser C2 of about 20 volts, al though each charge on condenser C2 after the first is slightly less than the preceding one. It should be important to note at this time that there is no resistance established across condenser C2 because it is desired that there be complete absence of current or charge leakage on this condenser during the voltage step-up opera tion.

Vacuum tubes A and B and E are normally non-conductive; that is, they are biased to cutoff condition. Tube A has approximately a 165 volt bias on its cathode, and hence the voltage across condenser C2 must exceed approximately volts before tube A conducts. The building up of a charge on C2 on the ninth rise to about 155 volts will cause tube A to start conducting very suddenly, as a result of which a pulse of current is passed through transformer T2, which is so poled that it applied a sharp positive pulse to the grid of tube B, causing tube B to conduct suddenly and pass a sharp pulse to the grid' of tube E via lead 5U. Tube B, in effect, is an overbiased pulse oscillator and is connected `regeneratively to produce only one pulse in response to the flow of current in tube A, after which it ceases conducting. Tube B triggers off or res to produce a constant amplitude discharge pulse irrespective of the amplitude of the pulse from tube A, and this discharge pulse causes normally non-conducting tube E to conduct. When tube E conducts, it provides a low impedance path for the charge on condenser C2. This condenser C2 then discharges through tube E to a relatively low value. The time constants of the counter or step generator I4 are so designed that there are nine risers or incremental steps in voltage on C2 before it discharges. The counter provides two outputs, one a step Wave voltage which is taken from condenser C2 and applied to lead 5I, and the other a synchronizing pulse at the end of each step wave which is taken from the transformer T2 and applied tclead I5. tion then repeats itself. The counter I4 can be arranged to provide a different number of steps by varying the ratio of the capacitance values of condensers CI and C2, if a different numberof channels are employed.

The cycle of opera-- from'the'countercircuit over lead 5I. .pling amplifier provides van output .from its 'cathode-.over lead 52 of the same polarity and voltage f lh'e coupling amplifier AHis merly'a'-cathode follower which obtainsfa-step wavevoltage input This `couas the inputwave taken from condenser C2,1without loading condenser C2. Amplier I6 does not draw grid current.

4Output yfrom the coupling amplieristaken from its cathode andis supplied to all'o'f'the channel selectors I 8 in parallel. The apparatus in` only one channelhas been shown since all'of the channels have substantiallyidentical.equipment and function similarly. Each Achannel includesa channel selector .I8 and the :different channel selectors are biased diierentlybyindividual cathode rheostats.53, ea'chof Whose eifective vvalues determines a vcertain cathode nbias. Each :channel selector includes a Vacuum tube 60,:a .series grid resistor lillllgan yadjustable cathode"'bias1resistor253,a pair of resistors 55 and58 connected in series between `the anode of .tube 5D. andthe source of `anode polarizing potential B, arbleeder resistor 51, and a pair of 'by-pass 4condensers59, 54.

The channel selectors are `normally non-conductive; i. e., biased to cut-off. Channel vnumber lisvfbiased to conduct on the first riset in .the applied step wave voltage, let us say at -{-`volts,

because the system is so arranged that condenser c C2 A.cannot discharge below Slet us say 30 volts. Thechannel selector in channel'number' isarrangedto conduct on the second rise at another I5 voltsv higher than .the bias on'the first channel selector; or at a value of l Volts. The succeeding channels `are .arranged to conduct/sequentially at different rises in the step Wave voltage applied to the different chan-nel selectors |:8 by the ycoupling Vamplifier I5. In the channel number three, current willflow in the `associated channel selector on #3 riser in the step wave, While in channel #6 currentzvvill start to flowin its associated channel selector on #.5 riserin the stepwave. It will thus beseen that the step am- .plitude at which each channel selector conducts is determined by the cathode bias which is developed across resistor 53 and-.condenser"54, and that diiferent adjustments of resistor 53 will `give different cathode biases.

VAfter each channel selector i8 which is normally non-conductive starts to conduct, 'further rises in the step wave will not produce any appreciable anode current increase in that channel selector because of grid current limiting due'jto resistor 49. Stated in other words, each channel selector is caused to carry current starting onthe vparticular step riser which furnishes a voltage of suiilcient magnitudeto overcome its cut-off potential, and this channel selector will continueto pass current to the end of the step wave voltage. Let it be assumed that channel selector lfor the first channel startsto conduct on the iirst rise in the step wave voltage at a value of 35 volts. When the second rise in ythe step wavevoltage occurs, thefirst channel selector is already lpassing current. The increase in voltage in the step voltage wave appearing in the output of the coupling amplier will merely cause the grid of the first channel selector to draw current, and the channel selector thus acts very much like laA diode between its grid and cathode to cause anode current limiting. Hence, as eachy addition- .allchannel selector after the first starts tolconduct (when additional rises occur `in the step awave) the preceding @channelA selectorsiwhich-are alreadyfconductlngfwill continue:conductingiwith substantially ithei'same fano'de `:current as before. After .thefninth rise'fin'ther step'i-wavezvoltageithe amplitudeof 'theastep wave ffalls' down to iitsfminimum value "(30volts in Vthis case) :and .all channel selectorsv will cease conducting. i'Ihis lcycle -zof operations `Will `repeat itself for each `new step wave voltage.

"The operation lof the :system .so far described in iconnection'with circuits .410, I2, .111,316 and l lmaybe better understoodfby'reference lto *the curveszl'll throughlM of Fig. P3. Curve99iillustrates'thefsinefwave outputof the 90Skc'. crystal oscillator JIU. SCurve llllus'trates the current pulsesfromthe anode circuit ofthe crystaloscillatorLlll. Curve 10| illustrates the short-sharp pulses initheloutput of the 9(1 kc. pulse oscillator liz-whichiare synchronized by thesine Wave oscillator. Curve |02 shows the steps'wave voltage produced'bythe counter 1M. It should -be noted that this step wave voltage begins at about "530 volts positive'which is the lowest voltage towhich condenser C2 can be discharged, and rises `to about '1155 volts positive. l",lhestep Wave voltage of curve |02 'is shown as having nine steps-or risers. Curve ID3 shows theanode voltage olf/one channelselector I8, let us'say in channel-number 4,.while curvevl'lMy shows thef-anode'Voltage-of sei lector I8 in channel l. Thus, referring to curve |03.' it Willbe seen that the anode voltage of the Vacuum tube60 of -the channel `selector'is ya maximum over a'period'tcorresponding to'the non-conducting period ofthe channel selector, and that this fano'de :voltage drops suddenly, on the fourth rise of thestep wave voltage curve 102 Whenthe channelfselector becomes conducting, and this anode voltage.- remains low over a period t'V corresponding to the conducting period of the channel selector of channel number 4. Similarly, by referring vto curve IDI, it will be seen thatthe anode voltage on the channel selector vacuum tube isa maximum. overaperiod t corresponding to the non-conducting period of the selector tube, and :drops suddenlyiat ythe rst rise in the step wave rvoltage of curve H12, 'at which `time the channel selector in channel #I becomes conducting. During the conductingperiod t of the channel selector I8 of channel #Lthe anode voltage on the channel selector tubewill remain low. It should be -no'ted'from curves YH33 and IM that once thechannel selector in a particular rchannel becomes conductive itcontinues'to be conductive to the end ofthe step wave of curve H12.

The channel selector I8 is so arranged that'the amplitude of the output yvoltage pulse on the anode of its vacuum tube 60 is constant regardless of its particular channel position; or,.put tingit in other Words,.the different Vchannel selectors furnish a constant amplitude output voltage over a'wide range of cathode biases for the'differentchannels without requiring additional :adjustments when its bias is changed over this range. The operating position of any one lchannel selector circuit I8 relative to the different risers on-.the step wave voltage can be changedby a simple bias adjustment Without the need for making other changes to compensate for the change in-bias. The reason for this will now-'be given.

If resistor 55 in the channel selector were-Zero, then vthe voltage at point 56 yWould be constant (equal to +B) regardless of channel position, but theganode-to-cathode voltage of the vacuum tube-60 .prior rto its conducting time would decrease withincreasinglchannel number (that-nis,

channel' position on the step Wave), since the voltage at the cathode increases as the channel number is increased (that is, as the channel position is change-d relative to the discharge of the step wave to thereby occupy a higher amplitude position on the step Wave), as a result of which the output voltages for the different channel selectors would be different. When resistor 55 is given a finite value, then the voltage at point 56 over the entire time for one complete step wave (considering only one cycle of operations) increases in the different channels with increasing channel number, since the average current in vacuum tube 6B and therefor in resistor 55 de creases over this same time interval. Stated otherwise, the voltage at point 56 increases as the bias on tube 6B is increased in order to provide for selection of a higher voltage level step on the applied step wave. By properly choosing a value of +B and resistor 55, the anode-tocathode voltage can be made constant with channel position for all channels; that is, changes of voltage at point 56 will be equal to changes in the cathode voltage as the channel position is changed, resulting in an amplitude of voltage pulse at the anode of tube 60 which is constant With channel position, since the peak anode current is a function of the anode to cathode voltage. Resistor 55 is a function of the anode voltage +B and is also a function of the amplitude of the input step Wave. Hence it will be appreciated that there is a wide range of values which resistor 55 may have to take. This value can easily be and has been found experimentally.

It should be evident that the resistors 55 in the different channels carry dilerent average currents, due to the different conducting periods of the channel selectors. Thus diierent average currents produce different IR drops in the resistors 55 to compensate for different rising cathode voltages, but the voltage outputs from the different channel selectors for a selected value of resistor 55 are practically the same since the cathode to plate voltage remains constant for all similar channel selector tubes 60. Hence, one channel can be used in another channel position Without making adjustments to compensate for the different cathode biases. The same value of resistor 55 is used in all channel selectors. Except for the diiferent adjustments in cathode biases, all of the channel circuits are identical.

A resistor 51 is employed in the channel selector to set the mean value of the voltage at point 56 in order to provide the proper operating conditions for vacuum tube 55. The use of this resistor A51 eliminates the necessity for varying the value of +B in order to obtain a suitable voltage for all channels. The resistor 51 serves to bleed current from the anode supply. This resistor 51 has the same adjustment or value in all channels. Condensers 54 and 59 are by-pass condensers which by-pass the alternating current component of anode current at the input frequency. Resistor 49 is a series grid resistor which, as mentioned before, serves to limit the grid voltage to Zero regardless of the applied voltage.

Although the channel selector |8 of the invention has been shown as having a resistor 51 shunting the by-pass condenser S, it should be understood that, if desired, the resistor 51 can be omitted, although this is not a preferred arrangement. To achieve the same resultslvvith the resistor 51 omitted, the value of B would have to be suitably selected in order to obtain 8. a desired voltage for all channels which would result in constant amplitude of voltage pulses.` from the channel selectors. The use of resistor- 51, however, eliminates the necessity for varyingV the value of B to obtain the desired voltage.

The output from each channel selector is in the form of a rectangular Wave pulse which is caused by the drop in anode voltage in the vacuum tube 60. This rectangular Wave output pulse is applied to the saw-tooth generator 2|! and is differentiated by condenser 6I and resistor 62. The result of this differentiation by condenser 6| and resistor 62 is a sharpnegative input which is applied to the grid of vacuum tube 9|. Tube 9| is normally conducting. The sharp negativev impulse applied to its grid from the differentiator 6|, 62 is of sufficient magnitude to bias the tubeI 9| to cut-off and beyond for the duration of the negative impulse.- The duration of this nega--` tive impulse is determined by the values of resistor 62 and condenser 6| and the amplitude of the rectangular pulse from tube 66. Since the amplitude of this rectangular pulse is constant in`- accordance with the invention, the duration of, this negative impulse is determined solely by the values of resistor 62 and condenser 6|.

During the time tube 9| is normally conducting, the voltage on its anode is low. The application of the negative impulse to the grid of tube 9| cuts off the flow of current in tube 9| and causes the anode voltage on this tube to rise, as a result of which condenser 63 starts to charge through resistor 65|. When tube 9| again conducts at the end of the applied negative pulse, condenser 63 discharges through the loW impedance path of tube 9| to its normal voltage. The result is a short saw-tooth wave obtainable from condenser 63 of a duration equal to the duration of the negative impulse applied to the grid of tube 9|. In practice, this saw-tooth Wave may be 10 psec. long, which is roughly of the order of a one channel interval, or the length of one tread of the step wave voltage (curve |52 of Fig. 3).

Curve |05 of Fig. 3 illustrates the wave form of the differentiated pulse in channel #4 which is applied to the grid of tube 9|. The interval t2 represents the non-conducting time dur-ation of tube 9|, corresponding to the modulation range of channel #4.

The horizontal dash line in curve |05 represents the cut-off grid voltage of the tube 9|. The maximum negative value of the differentiated pulse is about +30 volts.

The saw-tooth wave produced by saw-tooth generator 2E) in the output of tube 5| is applied to the pulse generator 22 and is stepped up or increased (from a direct current standpoint only) from an original +35 volts base line to about +50 volts base line at point 65. The sawtooth wave amplitude at point 65 is about 40 volts. Curve |66 in Fig. 3 illustrates the voltage wave form of the saw-tooth Wave applied to the pulse generator 22 at point 65.

The pulse generator 22, Whose grid is coupled to the output of the saw-tooth generator 20,' is normally non-conducting and is so lbiased as to start to conduct at about the middle of the applied saw-tooth wave in the absence of modulation. Coil 66, in the output of pulse generator vacuum tube 22, has voltages of relatively opposite polarity on its end terminals when a rapid current change occurs in it. This coil 'is a differentiating coil with distributed capacity. Damping resistors 61 are provided for both halves of the coil 66 so as to limit the oscillations in the coil' 51B: tofa; singleihalf period foreach separatechange of current- Curve |01 inFig. 3 illustrates the Wave form ofthe' anode--current ina pulse generator 22 of channel #41.for. the unmodulated condition. It should be noted that' the pulses in curve |01 commence'at about the middle of the applied saw-tooth wave of curvev |06. Curve |08 in Fig. 3 represents voltage pulses in the output of the pulsev generator 22 at point B8; due to the presence of current pulses of curve.|`|.

Modulation is applied to the Vacuum tube of pulsefgenerator 22.- by modulator tube 26 which seryesntoyarythe bias'ontube 22 and henceto vary the critical value or time at which the pulseY generatorl 22 starts. to :conduct over' the range ofthe :applied saw-tooth.

Vacuum; tube 69 isa cathode follower, as a result of which any variation in its grid voltage produces-.anequal voltagevariationon its cath- -odeiz Resistor carries the cathode currents offztubes-22, 26' and 69: Resistor 10 is a common'.` cathodel resistorv for both vacuum tubes 26T-and 69. A variation in current through tube $9.11inf response to modulation or rectified ringing causes av variation-*in the cathode voltage of tube 69;.. The' grid voltage of. tube 26 is fixedV and hence: the variation in cathode'voltage of tube 6.9;' changes the average anode response of the modulator: 26 and' henceV the cathode biasl of thee pulse: generator.:Y 22. Any` appreciable flow of. current through tubeA 22: must be through theimodulatorZi.` A veryk large resistor 1I'connected-between' groundi and, thel cathode of pulse generator4 22: (for:y example 2' megohms) is provided: to prevent the omission ofa pulse through the vacuum tulbe 22'; in then event themodulator tube.v 2.6 is cut off f by excessive modulation.

A failure to pass a pulse over eachv channel atfa'll times. for each assigned' periodY may result in''crossemodulation"or` crossetalk inthe channels due' to a change in power*levelsthroughout'the system; Each pulse` draws. about tvvo` amperes of current in the radio frequency transmitter and hen-ce4 the failure of' a pulse will change the-average current in thevarious circuits in the transmitter and tend to producev cross-modulationor-int'eraction of the channels.l The use of resistor 1|` prevents'the drop out of a pulse` in the" event of the foregoingcircuit conditions and assures the passage of aA pulse from the pulse generator for each channel. at all times during the? assigned periods;

Only-the front edge of the positive pulse passed byfpu-lse generator 22Y is ultimately used,` and this front edge varies in time over the selected range in accordance with the modulation.

The dotted line pulses in curve |08 on op` positesid'esof the solid line pulses of positive polarity. represents the extreme positiony during modulation; This extreme position is shown as covering av range t3, which is about i4 usec. onneach side of the solid line pulse, or 8f aseos. total.

The maximum advanced position in time due t'onegative modulated voltage is determinedby the' minimum anode resistance of modulator tube 26 (corresponding to maximum conduction position), which occurs when tube 59 is cut off; andthe actualfvalue of this minimum anode response-is set upby the values of resistors 1D and12': When amaximumpositive modulating voltage isz applied? to tube 69; its current will cut-off! tube 25: and leave' resistor 1| as the' biasing.v resistor for tube 221 of' thelpulse generator;

10 Thus.` the flowv of'. current through resistorl 1| when tube 26 is cutoff automatically biases the pulse generator 22 so that it yields a useful pulse atithetop position of any saw-tooth wave applied to the gridof'; tube 22V.

Resistor EZin-the saw-tooth generator 20 may lbe called the modulation limit control because itvariesthe duration ofW the negativel pulse in line |05 (Fig. 3) rand hence the' time during which., tubeT 9| of the` saw-tooth generator'- is cut-off. For this. reason, resistor 62 is shown asbeing variable.l

Speech and ringing are appliedv to leads113. Ringing is furnishedfby a twenty cycle low`fre; quencyaudible source-= (not shown). The condenser 14 in therinput circuit isla low reactance for. the. higher'speechfrequencies and isarelatively,- large reactancey forv the 10W` ringingvr fre.- quency. DuringI speechgthe modulationis applied: to the gridy of tube 69 Via transformerT3 and; lead 15: When'r ringing is.A applied, there is` developed a voltage across condenser 14 which is` applied to:r leads 16 to bev rectiedby tube 11, 13o-thereby develops,v direct current voltage across the. resistor 18, such: that a negative polarity appears on that terminal of resistor 1'8 which is connected-v to the grid of-'tube- 69 Tuber illis` cut off by thisV voltage of negative polarity resultingf from ringing, andVV will allowl modulator tube 26 to. assume its minimum-anode resistancel for f the flow of current through tubes 2i. andtZE;A 'I'he flnalresult of this isthat' the pulse; developed by the: pulse generator`l 22 oc@- curs at a time corresponding to a position nearest the beginningofY a saw-tooth-v Wave, henceadvancing: the timing' of the direct current pulse in the output or thel pulse generator 224 toY its extremeI position. Thisadvancedipulse Willzree main in this-f'position forlasA long as the'ringing 20 cycle'current israpplied to the particular chanl nel.

The' clipper-limiter 2'4" eliminates variation: of the amplitude'` of them'odulatedY pulse (note li'ne |08', Fig.'V 3) which appear atk point 68 and also discards or refuses to pass the negative ung modulated pulse of line |08, Fig. 3. Thislimiter prevents the' passage of" low. amplitude pulses which Would'contribute to cross modulation.

Theioutput from the clipper-limiter 24 is rep'-l resented by linev |09, Fig 3', which indicates the anode current of the' clipper-limiter 24 of' channel #'41 The dotted pulses in line |09 indicate the time variationsl in the pulse in this channel in response to modulation. Obviously, there -will lle-pulsesA like |09 for every channel at different positions along the time base, and these pulses in the dili'rent channels do not overlap.

All output pulses from the individual chan-'- nels to '8, inclusive, are applied to leadv 3|i which connects with thel common differentiator andlimiter circuit 32. Leadl is connected toa diiferentiator coil 19 which produces small duration pulses of positive and' negative polarities from the channel pulses on lead 3| as illustrated in line |10'. The positive portions ofthese pulses appearingin line |10 ofFig. 3 occur at thefront edgesoftlle applied pulses represented by line |99 in Fig. 3: These differentiated' pulses from coilV 151 (Fig. 3,? line H0) are appliedto a top and bottom normally non-conducting clipperelimiter The resulting pulses of current in the output of tube 8|] are fed into another differentiatorcoil 19', and the'l output from this coil" 15' fedtoanothertopA and bottom normally'non-con ducting. clipper1=-limiter. tube. 80; of."r circuit: 3l:

1l Coils 'I9 and I9 and tubes 8D and 8U', respectively, function in a similar manner. The dir'- ferentiated pulses from coil 'I9' which are impressed on the grid of the clipper-limiter tube 80' are shown in line I I I of Fig. 3.

The discharge pulse which is taken o the counter I4 and applied to lead I5 is shown in line II2 of Fig. 3. The positive section of the discharge pulse initiates the discharge of the step wave (Fig. 3 line |02) via tube E of counter circuit. The length of this discharge pulse is made adjustable by means ofthe RC constants of the circuit elements associated with tube B in the counter circuit. This discharge pulse is applied via lead I5 to the grid of vacuum 8| through differentiating condenser P.

The operation of the synchronizing pulse generator is as follows: On the negative going portion of the discharge pulse (line II2 of Fig. 3) tube 8l is cut off and remains cut off for a period of time determined by the time constants of condenser P and resistor P' (in the actual circuit tried out in practice this time was made'2 ,usec.). During the time when tube 8! is cut off a positive pulse is developed at point 82. It will now be seen that the occurrence time of the pulse at 82 is a function of the length of the discharge pulse of line I I2. Fig. 3, and the length of the pulse at 82 is a function of the time constants of P and P. .is then the synchronizing pulse and is coupled to the output transformer T4 via coupling tube 83. Coupling tube 83 is also a top and bottom limiter.

The positive going edge of the discharge pulse (line II2) has no effect on tube 82 since on the positive going edge P is charged up via grid of tube 8| and hence discharged on the negative or falling edge of the discharge pulse.

v. .The generated pulse at point 82 is applied to the grid of coupling tube 83, which is normally non-conducting due to the grid leak bias provided by resistor 85 and condenser 84. This posi- 2tive pulse on the anode of tube BI causes tube B3 to conduct and produce a pulse of current of the same shape as the voltage pulse applied to tube 83. Tube 83, it should be noted, draws its current through one winding of the transformer T4, which is common to both tubes 80 and 83. These common currents in tubes 80 and S3 give rise to a pulse of voltage in the output of transformer T4 at point B6 whose appearance is represented in line II4 of Fig. 3. Line II4, Fig. 3, illustrates the appearance of all the channel and synchronizing pulses which occur in the output of transformer T4. It will be seen that this synchronization pulse produced by the .synchronizing pulse generator is of longer duration' than the individual channel pulses and periodically occurs after every eighth channelV pulse. Vacuum tubes 8U and 8B' ofthe differentiators and limiters 32, 34 are both normally `non-conducting. Tube 80'V draws current each comprising three cascaded vacuum tube amplier The pulse at 82 f stagest The output of the power amplifier 36 at point 88 is shown in line I I5, Fig. 3, and is applied to the cathode of a magnetron oscillator 40 whose anode is grounded. The pulses at point 88 are direct current pulses of negative polarity having a time duration of .6 aseo. for the signal pulses and 2.2 aseo. for the synchronizing pulses. These direct current pulses are applied to the cathode of the magnetron and cause the magnetron to produce radio frequency pulses offa duration somewhat less than the duration of the pulses applied to the magnetron. The output from the magnetron is applied to a suitable antenna circuit via a concentric transmission line 89.

There is shown a suitable regulator circuit for stabilizing pulse amplitudevapplied to the magnetron and also a line coupling circuit in the output of the magnetron for correctly loading the magnetron for maximum power output.V

The radio frequency pulses from the magnetron, which are applied to the line 89 and hence to the antenna, may have a frequency of the order of 1000 megacycles or higher, and the duration of these pulses are somewhat shorter than the triggering or firing pulses applied thereto and occurring at point 88. The reason that the radio frequency pulses from the magnetron are of shorter duration than the direct current pulses applied thereto by the power amplifier is thatlthe rmagnetron does not oscillate until the crests of the applied pulses are almost reached. The radio frequency pulses may have a duration of about .4 aseo. for the signal pulses and 2.0 aseos. for the synchronizing pulses.

The term ground used in the specification and appended claims is deemed to include any point of zero or fixed radio frequency potential.

What is claimed is: l

1. In a' pulse type multiplex system having. `a plurality'of channels whose inputs are connected together in electrically parallel relationship, in each channel of which there is provided a chan'- nel selector vacuum tube, the channel selector vacuum tubes being differently biased to the anode current cut-oit condition so as to become conductive at different voltage levels of an applied input wave, each channel selector tube having an anode and a grid, a common source of anode potential for said tubes, and individual resistor networks connected between said anodes'and said source, each resistor network including a bypass condenser connected between ground and a point intermediate the ends of the resistor network, the method of operation which includes simultaneously applying an input wave of increasing voltage value to the inputs of said selector vacuum tubes, said input wave having such a range of voltage values that the different selector tubes become conductive at different voltage levels of the applied input wave, and selecting that same value for all of the anode resistor networks by varying the proportionate values of the resistor network and the common source of anode potential to produce pulses having the same amplitude of voltage from all of said selector tubes whenever they conduct regardless of the value of the cut-off bias thereon.

2. In a pulse type multiplex system having a plurality of channels whose inputs are connected together in electrically parallel relationship, in,

each channel of which there is provided a channel selector vacuum tube, the channel selector vacuum tubes being differently biased to the anode current cut-off condition to become conducy 13 tive at different voltage levels of an applied in-` putffwave, each channel selector tube having an anode and a grid, a common-source of anode potential for said tubes, and individual resistor networks connected between said anodes and said source, 'each resistor network including a bypass condenser shunted by a resistor and connected between ground and a point intermediate the ends of the resistor network, the method of operation which includes simultaneously applying a step voltage wave to the inputs of said selector tubes, said step voltage lwave having a plurality of risers of diier'entvoltage values such that diierent selector tubesbecome respectively conductive at different risers of the applied step wave, and selecting the same value for all of the anoderesistor networks by adjusting the value of said shunt resistor to produce pulses having the same amplitude of voltage from all of said selector tubes whenever they conduct regardless of the value of the cut-off bias.

WILLIAM D. HOUGHTON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

